Creep Rupture Process Research Articles

Simulation analysis of the evolution law of creep rupture crack extension in X-fractured rock body

Creep is the macroscopic manifestation of the process of generation, expansion, and penetration of microscopic cracks in a rock body. In this study, the GDEM continuous–discontinuous numerical simulation software was used to model a rock body containing X-fractured for the purposes of exploring creep crack expansion and rupture in the rock body, analyzing the effects of various factors on X-fractured the rock body under the rule of change of the creep curve, and assessing the influences of the intersection angle of the fracture and other factors on the non-parallel fractured rock body on the creep rupture process. The results show that an X-fractured rock body exhibits a mixed tensile–shear damage mode, with tensile damage being the main type of damage. In the isotropic creep stage of a rock body with X-fractured , the steady-state creep rate initially increases and then decreases as the sub- fracture length increases, with the change of the fracture angle of the creep rate of the w-type; the sub-fracture length of h is 14 mm, the rock body is the first to enter into the accelerated creep stage, for the different fracture intersection angle of the rock body For the rock mass with different fracture angles, the time sequence of entering the accelerated creep stage is consistent with the creep rate; when the fracture intersection angle is 45°, and the sub-fracture length is 12 mm, the rock mass has the largest degree of fragmentation, which has a significant impact on the creep damage; after using a single variable processing, it is found that the fracture intersection angle, the sub-fracture length and other factors compared to the fracture intersection angle has a greater impact on the creep damage of the X-fractured rock body. This paper can provide theoretical basis and reference for the study of rock engineering creep damage law and mechanical properties of X-fractured rock body.

Choice of complex equivalent stress for two different variants of the plane stress state

To describe the creep rupture process under the complex stress, various equivalent stresses have been considered. From them, the equivalent stress has been selected, at which the total error of the difference between the experimental and theoretical values takes the smallest value among the considered equivalent stresses. Three basic equivalent stresses have been considered, as well as two complex equivalent stresses, which are a linear combination of two basic ones with one material parameter. The analysis of the total errors in the considered experimental data has showed that with the simultaneous effect of internal pressure and axial force on the wall of tubular specimen (or biaxial tension of a plane element), an complex equivalent stress should be used in the form of a combination of the maximum normal stress and the Mises stress. At simultaneous torsion and tension of tubular specimen (or simultaneous tension and compression of a plane element), an complex equivalent stress should be used in the form of a combination of the maximum normal stress and the doubled maximum tangential stress.

Creep rupture in carbon nanotube-based viscoplastic nanocomposites

While the linear viscoelastic characteristics of polymer composites have been extensively studied, the nonlinear creep with progressive damage leading to the ultimate creep rupture in CNT-based viscoplastic nanocomposites still remains a challenging issue. In this paper, a unified approach is presented to study the whole lifetime of creep process including deformation, damage, and creep rupture. First, based on the concept of secant viscosity, a linear viscoelastic comparison composite is introduced to mimic the nonlinear viscoplastic nanocomposite, with the linear part further formulated in the Laplace transformed domain. Then, both progressive degradation of the interphase and creep damage of the polymer matrix are presented. A joint field-fluctuation method and work-rate equivalence is subsequently utilized to calculate the effective Mises stress and hydrostatic stress of the viscoplastic matrix. The principle of irreversible thermodynamics is then invoked to derive the thermodynamic driving force to characterize the evolution of creep damage and rupture process. After the theory is developed, it is demonstrated that the predicted effective creep strains of multi-walled CNT/polypropylene nanocomposites are calibrated by the experiments spanning over the whole three stages of primary creep, secondary creep, and tertiary creep. Both creep rupture time and rupture strain of the overall nanocomposite are found to increase with CNT volume concentration. It is concluded that population of CNT nanofillers can remarkably enhance the creep rupture resistance of CNT/polymer viscoplastic nanocomposites.

Selection of complex equivalent stress for two different variants of the plane stress state

To describe the creep rupture process under complex stress, various equivalent stresses are considered. From them, the equivalent stress at which the total error of the difference between the experimental and theoretical values takes the smallest value among the considered equivalent stresses is selected. In this paper, three basic equivalent stresses are considered, as well as two complex equivalent stresses, which are a linear combination of the basic ones with one material parameter. The analysis of the total errors in the considered experimental data shows that, with the simultaneous effect of internal pressure and the axial force on the wall of tubular specimens (or biaxial tension of a plane element), a complex equivalent stress should be used in the form of a combination of the maximum normal stress and the Mises stress. For simultaneous torsion and tension of tubular specimens (or simultaneous tension and compression of a plane element), a complex equivalent stress should be used in the form of a combination of the maximum normal stress and the doubled maximum tangential stress.

Investigating the stress level impact on the creep rupture behaviour of 2195-T84 Al-Li alloy: Experimental and constitutive modelling

In the present study, the creep rupture behaviour and microstructural evolution of 2195-T84 Al-Li alloy are investigated at different tensile stresses. It is found that as the applied stress during the creep rupture process increases, the corresponding creep strain and creep rate significantly increase. Moreover, the evolution of microstructures, including precipitates, dislocation density and creep cavities at different stages is characterized using a transmission electron microscope (TEM), X-ray diffractometer (XRD) and scanning electron microscopy (SEM). The main strengthening precipitates are identified as T1 (Al2CuLi) and θ′ (Al2Cu) phases. Obtained results show that as the applied stress increases, T1 and θ′ phases are gradually coarsened. This coarsening is more pronounced for θ′ phase. Furthermore, the creep cavities are mainly distributed at the interface between the insoluble second phase particles and the matrix, and their average sizes gradually increase as the applied stress increases. Meanwhile, the density and size of dimples on the fracture surface gradually decrease as the applied stress increases. Moreover, the main fracture mechanism changes from transgranular dimple fracture to quasi-cleavage fracture. Based on the microstructural evolution, a novel set of unified creep rupture constitutive model is proposed. The established model incorporates the evolution of microstructures, including the dislocation density, average length of T1 and θ′ precipitates and creep cavitation, and correlates microstructural variables with creep rate. The results calculated by the constitutive model are in good agreement with the experimental data, which validates the proposed model.

Structural evolutions during creep deformation of polyester industrial fiber via in situ synchrotron small-angle X-ray scattering/wide-angle X-ray scattering

This paper aims to identify the creep mechanisms of high tenacity (HT) polyester industrial fiber under different loads. In-situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) tests were conducted on a 1000 D HT fiber during the creep and creep recovery process with a low load (15 N) and a medium load (50 N) as well as creep rupture process with a high load (60 N). The measured creep strain-time curves comprised tensile zone (I), creep deformation zone (II) and creep recovery/rupture zone (III). The SAXS indicated that the macroscopic initial creep strain in zone I and creep deformations in zone II were attributed to conformation transition from gauche to trans in amorphous region, increasing the amorphous orientation and long period. Irreversible portion of conformation changes accounts for the small unrecoverable plastic creep strain after removing 15 N load in zone III. The initial creep strain in zone I and creep deformations in zone II of the 50 N creep process were bigger than the microscopic long period strain, because amorphous layers had conformational transformation and microfibril slippage which also produced a higher unrecoverable plastic creep strain in zone III. The long period increased significantly at the beginning of rupture zone with 60 N due to fragmentation of amorphous tie molecular. The disappearance of lamellar peaks at the end of rupture zone implied destruction of periodic lamellar structures, for the entire breakage of amorphous chains. The WAXS suggested that the crystal structure was stable under the creep loads.

The evolution behavior of second phases during long-term creep rupture process for modified 9Cr-1.5Mo-1Co steel welded joint

In this paper, evolution of Laves phase in weld metal (WM) during long-term creep exposure was investigated for modified 9Cr-1.5Mo-1Co steel welded joint (WJ) by microstructure characterization and theoretical calculation based on a series of creep tests at 620 °C. It was found that some rod-like Mo-rich Laves phase precipitated adjacent to M23C6 carbides on grain boundary and other isolated polygonal Laves phase particles formed in grains after long-term creep exposure. Number and size of Laves phase increased simultaneously at early stage of precipitation. The volume fraction nearly approached to equilibrium fraction while coarsening was still ongoing after 2500 h. Johnson-Mehl-Avrami and Ostwald ripening model indicated that precipitation of Laves phase was controlled by grain boundary diffusion with a decreasing rate. EDS results showed Laves phase in WM had the highest Mo content in the whole WJ. The pinning effects on grain boundaries by Laves phase became weaker during creep exposure due to its coarsening. More high angle boundaries and sub-structure in WM from EBSD results provided more potential sites for Laves phase nucleating. With Laves phase particles growth on grain boundaries in WM, solid solution elements like Mo in matrix were severely depleted, resulting in loss of solute strengthening effect and precipitation strengthening effect, thus leading to the rupture in WM.

Effect of Interaction Field-related Parameters on Creep Rupture Process in Lath Martensite Packet Structures based on FTMP

Effect of Interaction Field-related Parameters on Creep Rupture Process in Lath Martensite Packet Structures based on FTMP

FTMP-based New Visualization Scheme for inhomogeneous Recovery-triggered Creep Rupture Process

FTMP-based New Visualization Scheme for inhomogeneous Recovery-triggered Creep Rupture Process

Time-dependent micromechanical behavior in graphite/epoxy composites under constant load at elevated temperatures

Time dependent deformation at the individual fiber level was investigated in graphite fiber/epoxy composites at elevated temperatures using micro Raman spectroscopy (MRS) and a time dependent shear-lag based single fiber composite model (SFM). The modeling parameters were obtained from the creep response of the unfilled epoxy at several stress levels and at temperatures up to 80°C. An effective fiber spacing was used in the model predictions to account for the radial decay of the interfacial shear stress from the fiber surface. Good agreement was observed between the model predictions and MRS data when the temperature dependence of τp (the shear stress in the matrix yielded zone) and γc (the critical shear strain for the onset of inelasticity) were taken into account. Overall, the inelastic length growing from the fiber fractures increases with temperature and time. This leads to a wider stress concentration profile in the neighboring intact fibers, which increases the chance of failure in the intact fibers and facilitates the creep-rupture process of the composite.

Probabilistic micromechanical model of creep-rupture in filamentary composites

Abstract A model which accounts for the effect of interacting micromechanical damage mechanisms on the creeprupture process in filamentary composites is developed. Long-term results are extrapolated from short-term data by characterizing the progression of fundamental damage mechanisms. The dominant damage mechanisms considered in the model are the random creep-rupture of fibers and creep-debonding at the matrix/interface. The model predicts the scatter in the creep-rupture times of Kevlar/epoxy strands as well as the ‘accelerating’ failure rate observed in long-term tests. A parametric study of creep-rupture in carbon/ epoxy composites is also presented.

Acoustic emission and fiber damage in creep of a unidirectional [formula omitted] metal matrix composite at 450°C

Metal matrix composites (i.e., MMCs) have potentially for high temperature applications because of light weight, high stiffness and good heat resistance. Especially titanium alloy matrix composites reinforced with continuous SiC fibers are expected to have excellent applications at high temperature, and such MMCs have been tested under tensile, compressive and cyclic loadings to evaluate high temperature strength. The present authors performed creep tests of a unidirectional SCS-6/Ti-15-3 MMC at 450 C. They found that it exhibits creep deformation and rupture in longitudinal creep at stress levels much lower than the tensile strength though the reinforcing phase, SCS-6 by Textron, does not creep at the test temperature. This experimental result can be explained as follows: At high temperature, stress in the matrix is relaxed, and accordingly stress in the fibers is increased, as was analyzed by Goto and McLean. Then, fiber breakage may occur progressively, starting from the weakest fiber, and if a certain number of fibers are thus broken, overall rupture can be induced. It is obvious that this creep rupture takes place more easily under higher applied stress, and that it is promoted if the fibers suffer from damage such as cracking in the reaction zone, degradation of themore » coating layer, and so on. Monitoring of AE (i.e., acoustic emission) can be effective in verifying the creep rupture process mentioned above. Neu and Roman monitored AE during thermomechanical fatigue tests of a unidirectional SCS-6/TIMETAL[reg sign]21S MMC, and they showed how effective this method is. In the present work, first the authors monitored AE in creep tests of a 6-ply unidirectional SCS-6/Ti-15-3 MMC at 450 C. Then, to examine the damage evaluation based on AE monitoring, the authors observed fibers exposed by etching the outer matrix layers in the crept specimens.« less

High-temperature rupture of microstructurally unstable 304 stainless steel under uniaxial and triaxial stress states

Specimens of 304 stainless steel were tested to failure under two different stress states, uniaxial tension using smooth bar specimens and triaxial tension using notched bar specimens. The tests were conducted at a temperature that gives rise to carbide particle growth which, in turn, leads to microstructural softening. Rupture times are compared for uniaxial and triaxial stress states with respect to multiaxial stress parameters that are directly related to physical mechanisms. The success of the parameters is judged according to how well the rupture times of notched specimens can be predicted using the rupture data for specimens under uniaxial tension. The data indicate that the rupture time is not governed by deformation processes, despite evidence for substantial softening by particle coarsening. The results further suggest that the creep rupture process is dominated by cavitation that is coupled with localized shear deformation along the inclined grain boundaries.

Computational Simulation of the Creep-Rupture Process in Filamentary Composite Materials

A computational simulation of the internal damage accumulation which causes the creep-rupture phenomenon in filamentary composite materials is developed. The creep-rupture process involves complex interactions between several damage mechanisms. A statistically-based computational simulation using a time-differencing ap proach is employed to model these progressive interactions. The finite element method is used to calculate the internal stresses. The fibers are modeled as a series of bar elements which are connected transversely by matrix elements. Flaws are distributed randomly throughout the elements in the model. Load is applied, and the properties of the individual elements are updated at the end of each time step as a function of the stress history. The simulation is continued until failure occurs. Several cases, with different initial flaw dis persions, are run to establish a statistical distribution of the time-to-failure. The calcula tions are performed on a supercomputer. The simulation results compare favorably with the results of creep-rupture experiments conducted at the Lawrence Livermore National Laboratory.

Oxidation of a Mn‐Cr austenitic steel during creep testing and its interaction with the fracture

AbstractThe interaction between oxidation and creep rupture was studied in a 17 Mn‐10 Cr austenitic steel, of interest as structural material for the internal components of fusion reactors. The observation of the creep specimens tested in air at temperatures ranging from 773 to 973 K revealed the presence of an adherent oxide scale and of a ferritic phase underneath, which forms as a consequence of the Mn depletion of the austenitic matrix.The microstructure of the two layers was investigated by optical microscopy, SEM, X‐ray diffraction, EDS and magnetic permeability measurements. The scale has a complex structure, being composed mainly of manganese oxide. The ferritic layer is completely recrystallized and does not present grain boundary precipitates as the austenitic phase does.The effect of the surface modification on the creep rupture process is discussed in the light of a recent model of deformation‐oxidation interaction.

Creep rupture of polypropylene sutures as a function of diameter, radiation dose and temperature

The failure of polypropylene (PP) sutures was studied via creep rupture tests. Linear relationships were generated from plots of log time to break ( t B ) versus stress (σ). At a constant σ the results showed that t b was inversely proportional to the suture diameter (7-0,6-0,5-0 and 4-0) and to the radiation dose (0,15, 20, 25 and 50 Mrad). Moreover, for the 6-0 suture size at a 15 Mrad dose, ( t B ) decreased with increasing temperature (26, 37, 44, 50 and 60°C). For this creep rupture process the activation energy equalled 91.2 kJ/mol (21.8 kcal/mol). A mechanism for failure was presented which assumes that these sutures are bundles of oriented, semicrystalline microfibrils.

A creep-rupture model for two-phase eutectic solders

An analytical framework to predict failure of solders under creep conditions is proposed. A creep-rupture model for two-phase eutectic solders, based on both micromechanics and fracture mechanics, has been developed. The general agreement between the model predictions and reported creep-rupture data in the literature for lead/tin eutectic solder indicates that the mode and mechanisms proposed in the model may control the solder creep-rupture process. >

Fatigue and environmental resistance of polyester and nylon fibers

AbstractThe fatigue resistance of individual synthetic fibers can govern the performance of complex fiber assemblies such as tire cord and marine rope under certain loading conditions. This paper explores the relative performance of polyester and nylon 6,6 fibers and yarns, both dry and in aqueous solutions, primarily synthetic seawater. Fiber failure over a range of loading conditions and frequencies was found to occur at a critical cumulative strain, governed by a creep rupture process; the cyclic lifetime for both fibers is predictable using a simple creep rupture based theory. Polyester is more resistant to creep rupture, and consequently outperforms nylon 6,6 in cyclic fatigue. The advantage of polyester is considerably greater in aqueous solutions, where the performance of the nylon is diminished. Other comparisons indicate that the particular polyester fibers studied have higher stiffness and strength, lower strain to failure, and much lower hysteresis energy absorption compared with the nylon. The actual fatigue performance of complex fiber assemblies such as ropes is also limited under many conditions by factors not present in single fiber or yarn fatigue, including hysteric heating and internal and external abrasion.

Microstructure, creep, and tensile deformation in Ti-6Al-2Nb-1Ta-0.8Mo

The effects of microstructure, temperature, and stress level on the creep response of Ti-6211 have been investigated. A variety of microstructures simulating the heat affected zone of a weld (HAZ), as well as the as-received structure, were tested in a temperature range of 298 K to 873 K. At stress levels below the tensile yield strength, creep curves saturate in the ambient temperature regime. The colony type Widmanstatten alpha + beta as-received structure exhibited the highest creep strains at ambient temperatures. Long slip lengths associated with the large colony size, and sliding along various interfaces account for the relatively high creep strains to saturation. Planar arrays of straight dislocations operating on a single slip system were observed for samples crept at 298 K while thermally activated cross slip was observed for samples crept at 453 K. Beta-annealed martensitic micro-structures displayed enhanced creep resistance, out-performing other recrystallized HAZ structures. Above 778 K the activation energy of creep is close to that for self-diffusion in titanium, suggesting that diffusion-controlled dislocation mechanisms are the rate-controlling processes at elevated temperatures. Creep rupture at elevated temperatures occurred by microvoid nucleation and growth. Fracture occurred along colony boundaries in the as-received structure but appear to be intergranular with the crack propagating along G. B. alpha/matrix interfaces in the equiaxed microstructures. Sliding along alpha/beta interfaces, colony boundaries, prior beta grain boundaries, and slip traces contributed to the creep strain and rupture process. Cyclic creep with a loading-unloading sequence was also performed at room temperature and cyclic creep acceleration was observed.

Modelling of the coupled effect of plastic damage and creep damage in Nimonic 80a

The title problem was discussed from a viewpoint of continuum damage mechanics. By assuming that both the plastic and the creep damages are governed by the formation and the growth of the grain boundary cavities, the states of plastic and creep damages caused by these cavities were first represented in terms of a symmetric second-rank damage tensor. The interaction between these two kinds of damage was formulated by establishing the evolution and the constitutive equation of the damaged materials on the basis of the experimental observations on microscopic mechanisms of these damages and their mechanical effects reported so far. Finally, the effects of material damage induced by prior plastic strain in tension, compression and torsion at room temperature on the subsequent creep and creep rupture process of Nimonic 80A at 750°C were analysed by these equations, and the validity and the limitations of the proposed theory were discussed by comparing the numerical results with the corresponding experimental ones. It was observed that the proposed theory not only described well the significant reduction of creep strength and creep rupture time brought about by the preceding plastic deformation at room temperature, but it also represented adequately the anisotropic features of plastic and creep damages.